Update guides with new mongoDB version requirement (Parse Server 4.0) (#715)

* change min mongo version to 3.6

* change min mongo version requirement

Author: TomWFox

_includes/parse-server/database.md

@@ -10,7 +10,7 @@ If you have not used MongoDB before, we highly recommend familiarizing yourself
 
 The Mongo requirements for Parse Server are:
 
-* MongoDB version 2.6.X, 3.0.X, or 3.2.X
+* MongoDB version 3.6
 * An SSL connection is recommended (but not required).
 
 If this is your first time setting up a production MongoDB instance, we recommend using either [mLab](http://www.mLab.com) or [ObjectRocket](https://objectrocket.com/). These are database-as-a-service companies which provide fully managed MongoDB instances, and can help you scale up as needed.

_includes/parse-server/getting-started.md

@@ -10,7 +10,7 @@ Parse Server is an open source version of the Parse backend that can be deployed
 **Prerequisites**
 
 * Node 8 or newer
-* MongoDB version 2.6.X, 3.0.X or 3.2.X
+* MongoDB version 3.6
 * Python 2.x (For Windows users, 2.7.1 is the required version)
 * For deployment, an infrastructure provider like Heroku or AWS